How does convection relate to the atmosphere?
Convection is caused by unequal heating of the atmosphere. Usually this is caused by the unequal heating of the surface of the Earth (due to albedo) which then heats the air above it.
Albedo is the reflectiveness of an object. Some objects will reflect more light/energy than other objects. Black objects reflect almost no light (that is why they are black) where white objects reflect almost all light.
If you have an area of freshly tilled farmland (black soil), next to a field of crops the exposed black soil will absorb more solar energy than the green plants which reflect some of the sunlight (specifically the green wavelengths). Therefore the black soil will heat up more than the crops. The black soil will therefore radiate more sensible heat into the atmosphere which means the air above the soil gets warmer than the air above the crops. And since air parcels do not readily mix, the temperature does not even out.
As per Charles's Law, temperature and volume are proportional, so as the temperature goes up the volume goes up. The parcel of air that is warmer will expand. Since the volume goes up in the parcel of air the pressure drops (Boyle's law) and the warmer air begins to rise. This rising will continue until the temperature of the surrounding air equals or exceeds the temperature of the rising air. That is how convection works in the atmosphere.
If you would like to know more about convection, like how convective type clouds are formed, please ask.